docs: def order_by_first_occurrence(self):
          """
          :return: The event in order of minimum occurrence. 
          """
          def _key(e):
              try:
                  return e.occurrence_list[0].start
              except IndexError: # no occurrences; put last
                  return localize(datetime.max - timedelta(days=365))
  
          return sorted(list(self), key=_key)